1. Field of the Invention
The present invention relates to a composite electrode for a secondary battery and a production method thereof and more particularly, to a composite electrode for a rechargeable battery that improves the capacity, cycling performance, and power density, and a production method of the electrode.
2. Description of the Prior Art
In recent years, various research and development have been vigorously performed for the purpose of realizing a light-weight, high output-density secondary battery using the electrochemical doping and dedoping property of a conducting polymer such as polyacetylene and polyaniline.
An example of the conventional secondary batteries of this sort uses polyaniline (Pan) as a positive electrode and metallic lithium (Li) as a negative electrode. In this secondary battery, the reactions at the positive and negative electrodes (i.e., electrode reactions) during the charging process are expressed in the following chemical equations (1a) and (2a) respectively. EQU Pan+nClO.sub.4.sup.- +nH.sup.+.fwdarw.Pan(ClO.sub.4).sub.n +ne.sup.- +nH.sup.+ (1a) EQU Li.sup.+ +e.sup.-.fwdarw.Li (2a)
During the discharging process, the electrode reactions are expressed in the following chemical equations (1b) and (2b), respectively, which are reverse of the equations (1a) and (2a). EQU Pan+nClO.sub.4.sup.- +nH.sup.+.rarw.Pan(ClO.sub.4).sub.n +ne.sup.- +nH.sup.+ (1b) EQU Li.sup.+ +e.sup.-.rarw.Li (2b)
As seen from the equations (1a) and (1b), in the reactions at the positive electrode, the doping and dedoping processes of a dopant anion (here, ClO.sub.4.sup.31) are used as the battery reaction, i.e., redox reaction.
In the equations (1a) and (1b), n is the doping rate of the dopant anion, ClO.sub.4.sup.-. It has been known that the doping rate n is equal to 0.5 or less, in other words, the maximum value of n is 0.5 (=50%) for polyaniline.
The capacity C (mAh/g) of the active material (i.e., polyaniline) of the positive and negative electrodes are calculated by the following equation (3) ##EQU1##
where N.sub.elec is the number of reaction-participant electrons and M.sub.r is the molecular weight of the active material (i.e., polyaniline). Since polyaniline is a polymer material, the molecular weight of the monomer unit of polyaniline is used as the molecular weight M.sub.r.
The monomer unit of polyaniline has a molecular weight M.sub.r of 92g. If the doping rate n of polyaniline is supposed to be 0.5 (=50%), the number N.sub.elec of reaction-participant electrons is equal to 0.5 In this case, the capacity C (mAh/g) of the active material (i.e., polyaniline) is given as 144 mAh/g from the equation (3).
In general, the doping and dedoping reactions (i.e., redox reaction) of the conducting polymers occur with good reversibility, the reason of which is as follows.
Specifically, the matrix of the conducting polymers has a more flexible structure than that of the inorganic materials. Therefore, the volumetric increase and decrease of the matrix will occur with good reversibility during the doping and dedoping processes of the dopant into the matrix.
The conventional secondary batteries using a conducting polymer such as polyaniline as the active material of the positive electrode have the following problems.
A first problem is that even if the active material (i.e., conducting polymer such as polyaniline) has a sufficient reversibility, the cycling property of the battery or positive electrode itself tends to disappear.
The reason of the first problem is explained below with reference to FIG. 1 illustrating a partial cross-section of the positive electrode of the conventional battery.
In FIG. 1, the positive electrode 106 is comprised of a plate-shaped conductive collector 101 and an active material layer 105. The active material layer 105 is fixed in contact with an opposing surface of the collector 101. The active material layer 105 contains a particulate active material (i.e., conducting polymer such as polyaniline) 102 and a particulate conductivity-imparting agent (e.g., carbon) 103, both of which are combine with a binder (not shown) to have a layer-shaped structure. A liquid electrolyte 104 containing LiClO.sub.4 is permeated into the miniaturized pores of the active material layer 105.
The battery or redox reaction occurs between the active material 102 and the electrolyte 104 and thus, electrons are transferred between active material 102 and the electrolyte 104 through the collector 101 or the conductivity-imparting agent 103. Therefore, to realize a satisfactory cycling property of the electrode 106, no only the reversibility of the redox reaction of the active material (i.e., conducting polymer) 102 but also the electron conductivity among the conductivity-imparting agent 103, the collector 101, and the active material 102 need to be ensured.
When one of the known conducting polymers (e.g., polyaniline) is used as the active material 102, some volumetric increase or decrease (i.e., expansion or shrinkage) of the active material 102 will occur together with the battery reaction or the charging/discharging reactions. Specifically, when the dopant anion (e.g., ClO.sub.4.sup.-) is doped into the matrix of the active material (e.g., polyaniline) 102 to charge the battery, the volume of the active material 102 tends to expand. When the dopant anion (e.g., ClO.sub.4.sup.-) is dedoped from the matrix of the active material (i.e., polyaniline) 102 to discharge to battery, the volume of the active material 102 tends to shrink.
On the other hand, no expansion nor shrinkage occurs in the collector 101 during the charging and discharging processes. Accordingly, the contact resistance tends to increase at the contact areas of the active material layer 105 and the collector 101, thereby degrading or preventing the electron conductivity.
As a consequence, the cycling property of the battery or positive electrode 106 itself tends to disappear while the reversibility in the redox reaction of the active material 102 is kept sufficiently.
A second problem is that the capacity per volume is small. The reason is that the active material layer 105 using the conducting polymer such as polyaniline has a low density and therefore, the capacity per volume of the layer 105 becomes smaller than that of the known inorganic active material layers.
The capacity per weight and the capacity per volume of polyaniline, and LiCoO.sub.2 and LiMn.sub.2 O.sub.4 as examples of the inorganic active materials are listed in the following Table 1.
TABLE 1 CAPACITY PER CAPACITY PER ACTIVE DENSITY WEIGHT VOLUME MATERIAL (g/dm.sup.3) (mAh/g) (mAh/dm.sup.3) LiCoO.sub.2 5.1 137 698 LiMn.sub.2 O.sub.4 4.3 104 447 POLYANILINE 1.3 144 187
In Table 1, the following relationship is established as EQU (CAPACITY PER VOLUME)=(CAPACITY PER WEIGHT).times.DENSITY.
As seen from Table 1, although the capacity per weight of polyaniline is approximately equal to that of LiCoO.sub.2 and LiMn.sub.2 O.sub.4, the capacity per volume of polyaniline is smaller than that of LiCoO.sub.2 and LiMn.sub.2 O.sub.4 due to smallness of the density.
A third problem is that the power density is low.
The power density of the battery or electrode 106 is determined by the reaction rate of the redox reaction of the active material 102 and the diffusion rate of the reaction-participant ion in the liquid electrolyte 104. Since the diffusion rate of the reaction-participant ion is typically lower than the reaction rate of the redox reaction of the active material 102, the power density is dominated by the diffusion rate of the ions.
The diffusion rate of the reaction-participant ion in the electrolyte 104 increases with the decreasing radius of the ions. Therefore, it is preferred that the reaction-participant ion has a radius as small as possible.
In the previously-described equations (1a), (1b), (2a), and (2b) where the ClO.sub.4.sup.31 ions with a comparatively large radius are used as the reaction-participant ion, i.e., the dopant anion, the power density becomes low due to the low diffusion rate of the dopant anion.